bus: fix capabilities on big-endian

The kernel provides capabilities as a u32 array, sd-bus uses an u8 array.
This works fine on little-endian as both are encoded the same way.
However, this fails on big-endian if we do not perform sufficient
byte-swapping on each u32 entry.

This patch makes sd-bus use u32, too. We avoid changing any kernel
provided data so we can keep pointing into kdbus pool buffers which
contain u32 arrays.